Interaction of tripodal Schiff-base ligands with silver(i): structural and solution studies

Abstract

The binding and extraction of Ag(I) by tripodal Schiff-base ligands incorporating different aromatic podand arms are reported. These ligands have been synthesized by condensation of tris(2-aminoethyl)amine with benzaldehyde (1), 4-phenylbenzaldehyde (2), 2- (3) and 4-pyridinecarbaldehyde (4). The structures of 1 and of four Ag(I) complexes [Ag(1)]ClO₄, [Ag(2)]ClO₄, [Ag₃(3)₂](ClO₄)₃ and {[Ag₃(4)₂](ClO₄)₃}_n have been determined by single crystal X-ray diffraction. The structure of 1 shows intramolecular C–H⋯π (aromatic) interactions between phenyl rings, while bonding of Ag(I) via Ag–N interactions within the ionophore pocket is confirmed. In [Ag(1)]ClO₄, and [Ag(2)]ClO₄ additional long-range Ag–H interactions are observed, while π–π stacking occurs in the polynuclear species [Ag₃(3)₂](ClO₄)₃ and {[Ag₃(4)₂](ClO₄)₃}_n. Potentiometric titration, liquid–liquid extraction and ¹H NMR spectroscopic studies were performed to probe the nature of the silver complexes in solution. Potentiometric studies confirm increasing complex stability with Ag(I) in the order 2 < 4 < 1 < 3, and enhanced Ag(I) extraction efficiency was observed with both increasing lipophilicity of the ionophore and the presence of additional donor groups. ¹H NMR spectroscopic studies were employed to probe the solution complexation behaviour of 1–4 towards Ag(I) and these confirm the formation of primarily 1 : 1 Ag : L complexes in solution.